The invention relates to receivers, and more particularly to a pulsating noise eliminating device in such a receiver.
In a receiver, especially in an FM receiver intended for automobile use, pulsating electrical noise emanating from the spark plugs of the vehicle and noise generated by the wiper often are a problem. Such pulsating noise can be eliminated to some extent by passing the received signal through a limiter circuit following intermediate frequency amplification. However, this technique for eliminating the pulsating noise is not entirely satisfactory. Accordingly, in order to more completely eliminate the pulsating noise, a further noise eliminating circuit 10 as shown in FIG. 1 has been employed at the output of the FM detector circuit in the receiver.
Specifically, an FM detected composite signal including a 19 KHz pilot signal formed at the output of an FM detector circuit 1 is applied to a pulsating noise detection circuit 3 which includes a high-pass filter which detects the presence of the noise. The output of noise detection circuit 3 triggers pulse shaping circuit 4. The shaping circuit 4 includes a monostable multivibrator which provides as its output control pulses of a predetermined duration.
The composite signal after passing through the delay circuit 2 is applied to a switching circuit 5 the on-off operation of which is controlled by output pulses from the monostable multivibrator 4. The switching circuit 5 operates to prevent the composite input signal from being applied to the next stage when the output pulses from the monostable multivibrator 4 so indicate. Accordingly, during such periods, the output of a level hold circuit 6 holds the level of the composite signal on the output switching circuit 5 at the level which it had immediately before the switching circuit 5 interrupted the signal. The outputs of the switching circuit 5 and the hold circuit 6 are applied to a multiplex demodulation circuit 8 which separates the signal into right and left channel signals.
FIGS. 2a-2c show waveform diagrams useful for more fully describing the operation of the circuitry shown in FIG. 1. FIG. 2a shows the FM detection signal including a 19 KHz pilot signal. If a pulsating noise signal (not shown) were to be superposed on the signal during the time period from the time instant t.sub.1 on the time instant t.sub.2, the monostable multivibrator 4 would be activated by the output of the high-pass filter 3 so that an inhibition pulse would be produced for a predetermined period T. Accordingly, during the period T, the switching circuit 5 remains in the interrupting state so that the signal is not transmitted. The signal level at the time instant t.sub.1 which is held by the level hold circuit 6 is thus transmitted. As a result, a signal as shown in FIG. 2b is applied to the demodulation circuit 8. The signal obtained after the signal of FIG. 2b has passed through a low-pass filter (not shown) in the demodulation circuit 8 is as shown in FIG. 2c. Thus, the pulsating noise signal during the period T is eliminated. However, should the audio signal be very small in comparison with the 19 KHz pilot signal, a noise signal proportional to the level of the pilot signal would be generated during the period T.
In order to overcome this difficulty in one technique which has previously been employed, the 19 KHz pilot signal is completed during the non-conductive period of the switching circuit by use of a resonance circuit or an oscillator circuit. A second technique has been to remove the pilot signal by using a 19 KHz trap circuit or notch filter in the stage prior to the switching circuit while the 19 KHz signal needed for the multiplex demodulation circuit is applied thereto directly through a bypass around the switching circuit.
However, the first of these techniques is disadvantageous in that channel separation is degraded since a 19 KHz resonance circuit or oscillator circuit must additionally be provided in the signal path thereby distorting the frequency characteristics of the entire circuit near 19 KHz and shifting the phase of the signal. The second technique is also disadvantageous in that, as the audio information signal (15 KHz-23 KHz) in the vicinity of 19 KHz is affected, the frequency characteristics are distorted and the signal's phase is shifted.